The present invention relates to the thermal treatment of a substrate having multiple layers, according to which a substrate layer that is covered from above and below is oxidized from the side edges thereof toward the center.
Such a method is known, for example, during the production of an aperture for surface emitting semiconductor lasers having a vertical resonator, which is also designated as a Vertical Cavity Surface Emitting Laser (VCSEL). A VCSEL is a semiconductor laser in which the radiation that is produced is propagated vertically, i.e. in a direction perpendicular to the semiconductor surface and a p-n-junction plane. With conventional lasers, the propagation of the radiation is effected in a direction parallel to the semiconductor surface and the p-n-junction plane. VCSEL's are used as the preferred light sources for large parallel optical communication architectures. To enable a coupling of the VCSEL's with optical fibers, they must emit a Gaussian radiation profile. For the Gaussian radiation profile, and for a laser activation, a special circular aperture is necessary in the VCSEL, and it must be defined precisely.
In the past, this aperture was formed by an oxidation of AlGaAs structures of the VCSEL's that progressed from the side. For the oxidation, the VCSEL's in a process chamber were brought to a treatment temperature and were oxidized with a nitrogen-containing water vapor. After the oxidation, the VCSEL's in the process chamber were cooled in an inert gas atmosphere to a temperature lower than the process temperature, and they were subsequently removed from the process chamber. In order to increase the throughput, with many VCSEL processes, the substrates are already removed from the process chamber at process temperatures, with the cooling of the substrates then generally being effected in ambient air.
The oxidation from the side, as is known for example from U.S. Pat. No. 6,014,400, is initiated by the nitrogen-containing water vapor treatment, and an oxidation front is formed that proceeds from the outside toward the interior. During the cooling-off in inert gas atmospheres, and during removal of the wafers from the process chamber, or during the cooling-off of the substrates having VCSEL structures in ambient air, a progression of the oxidation is gradually retarded and is brought to a stop. This retardation and stopping of the progress of the oxidation front can, however, not be precisely controlled with the above-mentioned method, so that it is not possible to form a good and predictable aperture.
For good optical properties of the VCSEL's, especially for the formation of a desired spatial radiation profile, such as for example a Gaussian radiation profile, a precise aperture formation is, however, necessary.
Proceeding from the above described state of the art, it is therefore an object of the present invention to provide a method with which a laterally progressing oxidation of a layer of a multi-layer substrate is possible such that a defined central portion is not oxidized, i.e. that a progression of the lateral oxidation can be stopped at a specified point.